Cache region concept

ABSTRACT

A method to associate a storage policy with a cache region is disclosed. In this method, a cache region associated with an application is created. The application runs on virtual machines, and where a first virtual machine has a local memory cache that is private to the first virtual machine. The first virtual machine additionally has a shared memory cache that is shared by the first virtual machine and a second virtual machine. Additionally, the cache region is associated with a storage policy. Here, the storage policy specifies that a first copy of an object to be stored in the cache region is to be stored in the local memory cache and that a second copy of the object to be stored in the cache region is to be stored in the shared memory cache.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/024,554, filed Dec. 28, 2004, entitled “CACHE REGION CONCEPT,” whichis incorporated herein by reference.

TECHNICAL FIELD

Embodiments of the present invention relate to memory management, and inone embodiment, a method to minimize memory footprint of differentsoftware entities and maximize performance using already constructedobjects.

BACKGROUND

FIG. 1 shows a prior art computing system 100 having N virtual machines113, 213, . . . N13. The prior art computing system 100 can be viewed asan application server that runs web applications and/or business logicapplications for an enterprise (e.g., a corporation, partnership orgovernment agency) to assist the enterprise in performing specificoperations in an automated fashion (e.g., automated billing, automatedsales, etc.).

The prior art computing system 100 runs are extensive amount ofconcurrent application threads per virtual machine. Specifically, thereare X concurrent application threads (112 ₁ through 112 _(x)) running onvirtual machine 113; there are Y concurrent application threads (212 ₁through 212 _(Y)) running on virtual machine 213; . . . and, there are Zconcurrent application threads (N12 _(t) through N12 _(Z)) running onvirtual machine N13; where, each of X, Y and Z are a large number.

A virtual machine, as is well understood in the art, is an abstractmachine that converts (or “interprets”) abstract code into code that isunderstandable to a particular type of a hardware platform. For example,if the processing core of computing system 100 included PowerPCmicroprocessors, each of virtual machines 113, 213 through N13 wouldrespectively convert the abstract code of threads 112 ₁ through 112_(X), 212 ₁ through 212 _(Y), and N12 ₁ through N12 _(Z) intoinstructions sequences that a PowerPC microprocessor can execute.

Because virtual machines operate at the instruction level they tend tohave processor-like characteristics, and, therefore, can be viewed ashaving their own associated memory. The memory used by a functioningvirtual machine is typically modeled as being local (or “private”) tothe virtual machine. Hence, FIG. 1 shows local memory 115, 215, N15allocated for each of virtual machines 113, 213, . . . N13 respectively.

A portion of a virtual machine's local memory may be implemented as thevirtual machine's cache. As such, FIG. 1 shows respective regions 116,216, . . . N16 of each virtual machine's local memory space 115, 215, .. . N15 being allocated as local cache for the corresponding virtualmachine 113, 213, . . . N13. A cache is a region where frequently useditems are kept in order to enhance operational efficiency.Traditionally, the access time associated with fetching/writing an itemto/from a cache is less than the access time associated with otherplace(s) where the item can be kept (such as a disk file or externaldatabase (not shown in FIG. 1)).

For example, in an object-oriented environment, an object that issubjected to frequent use by a virtual machine (for whatever reason) maybe stored in the virtual machine's cache. The combination of the cache'slow latency and the frequent use of the particular object by the virtualmachine corresponds to a disproportionate share of the virtual machine'sfetches being that of the lower latency cache; which, in turn,effectively improves the overall productivity of the virtual machine.

A problem with the prior art implementation of FIG. 1, is that, avirtual machine can be under the load of a large number of concurrentapplication threads; and, furthermore, the “crash” of a virtual machineis not an uncommon event. If a virtual machine crashes, generally, allof the concurrent application threads that the virtual machine isactively processing will crash. Thus, if any one of virtual machines113, 213, N13 were to crash, X, Y or Z application threads would crashalong with the crashed virtual machine. With X, Y and Z each being alarge number, a large number of applications would crash as a result ofthe virtual machine crash.

Given that the application threads running on an application server 100typically have “mission critical” importance, the wholesale crash ofscores of such threads is a significant problem for the enterprise.

SUMMARY

Methods for a treatment of cached objects are described. In oneembodiment, an application is configured to cache objects according todifferent caching treatments, each different caching treatmentassociated with its own region of cache. The configuring also involvesinstalling a different combination of storage and eviction policyplug-ins for each different region of cache, with each of the storageplug-ins including code to perform get and put operations, and each ofthe eviction policy plug-ins including code to perform cache evictionoperations.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and notlimitation, in the figures of the accompanying drawings in which:

FIG. 1 shows a portion of a prior art computing system.

FIG. 2 shows a portion of an improved computing system.

FIG. 3 shows a cache management service.

FIG. 4 illustrates one embodiment of a cache implementation with respectto local memory and shared memory.

FIG. 5 illustrates an embodiment of a first cache region flavor.

FIG. 6 illustrates an embodiment of a second cache region flavor.

FIG. 7 illustrates an embodiment of a third cache region flavor.

FIG. 8 illustrates an embodiment of a fourth cache region flavor.

FIG. 9 illustrates one embodiment of different programming models for astorage plug-in.

FIG. 10 illustrates one embodiment of an organization structure of acache region.

FIG. 11 illustrates a block diagram of one embodiment of a “get”operation using the Group Manipulation functionality.

FIG. 12 illustrates a detailed perspective of retrieving an attributeassociated with a particular object.

FIG. 13 a illustrates an embodiment of an eviction policy plug-in.

FIG. 13 b illustrates a detailed perspective of various types of queuesthat may be implemented by the Sorting component of an eviction policyplug-in.

FIG. 14 illustrates a detailed graph of one type of Eviction timingcomponent functionality.

FIG. 15 illustrates a detailed graph of another type of Eviction timingcomponent functionality.

FIG. 16 shows a depiction of a cache region definition building process.

FIG. 17 illustrates a detailed perspective of one embodiment of adistributed cache architecture.

FIG. 18 illustrates a block diagram of one method of sharing an objectbetween different computing systems.

FIG. 19 illustrates an embodiment of a computing system.

DETAILED DESCRIPTION

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone skilled in the art that the present invention may be practicedwithout some of these specific details. In other instances, well-knownstructures and devices are shown in block diagram form.

Note that in this detailed description, references to “one embodiment”or “an embodiment” mean that the feature being referred to is includedin at least one embodiment of the invention. Moreover, separatereferences to “one embodiment” in this description do not necessarilyrefer to the same embodiment; however, neither are such embodimentsmutually exclusive, unless so stated, and except as will be readilyapparent to those skilled in the art. Thus, the invention can includeany variety of combinations and/or integrations of the embodimentsdescribed herein.

The present invention includes various steps, which will be describedbelow. The steps of the present invention may be performed by hardwarecomponents or may be embodied in machine-executable instructions, whichmay be used to cause a general-purpose or special-purpose processorprogrammed with the instructions to perform the steps. Alternatively,the steps may be performed by a combination of hardware and software.

The present invention may be provided as a computer program product thatmay include a machine-readable medium having stored thereoninstructions, which may be used to program a computer (or otherelectronic devices) to perform a process according to the presentinvention. The machine-readable medium may include, but is not limitedto, floppy diskettes, optical disks, CD-ROMs, and magneto-optical disks,ROMs, RAMs, EPROMs, EEPOMs, magnetic or optical cards, flash memory, orother type of media/machine-readable medium suitable for storingelectronic instructions.

Shared Memory and Shared Closures

FIG. 2 shows a computing system 200 that is configured with lessapplication threads per virtual machine than the prior art system ofFIG. 1. Less application threads per virtual machine results in lessapplication thread crashes per virtual machine crash; which, in turn,should result in the new system 200 of FIG. 2 exhibiting betterreliability than the prior art system 100 of FIG. 1.

According to the depiction of FIG. 2, which is an extreme representationof the improved approach, only one application thread exists per virtualmachine (specifically, thread 122 is being executed by virtual machine123; thread 222 is being executed by virtual machine 223, . . . and,thread M22 is being executed by virtual machine M23). In practice, thecomputing system 200 of FIG. 2 may permit a limited number of threads tobe concurrently processed by a single virtual machine rather than onlyone.

In order to concurrently execute a comparable number of applicationthreads as the prior art system 100 of FIG. 1, the improved system 200of FIG. 2 instantiates more virtual machines than the prior art system100 of FIG. 1. That is, M>N.

Thus, for example, if the prior art system 100 of FIG. 1 has 10application threads per virtual machine and 4 virtual machines (e.g.,one virtual machine per CPU in a computing system having four CPUs) fora total of 4×10=40 concurrently executed application threads for thesystem 100 as a whole, the improved system 200 of FIG. 2 may only permita maximum of 5 concurrent application threads per virtual machine and 6virtual machines (e.g., 1.5 virtual machines per CPU in a four CPUsystem) to implement a comparable number (5×6=30) of concurrentlyexecuted threads as the prior art system 100 in FIG. 1.

Here, the prior art system 100 instantiates one virtual machine per CPUwhile the improved system 200 of FIG. 2 can instantiate multiple virtualmachines per CPU. For example, in order to achieve 1.5 virtual machinesper CPU, a first CPU will be configured to run a single virtual machinewhile a second CPU in the same system will be configured to run a pairof virtual machines. By repeating this pattern for every pair of CPUs,such CPU pairs will instantiate 3 virtual machines per CPU pair (whichcorresponds to 1.5 virtual machines per CPU).

Recall from the discussion of FIG. 1 that a virtual machine can beassociated with its own local memory. Because the improved computingsystem of FIG. 2 instantiates more virtual machines that the prior artcomputing system of FIG. 1, in order to conserve memory resources, thevirtual machines 123, 223, . . . M23 of the system 200 of FIG. 2 areconfigured with less local memory space 125, 225, . . . M25 than thelocal memory space 115, 215, . . . N15 of virtual machines 113, 213, . .. N13 of FIG. 1. Moreover, the virtual machines 123, 223, . . . M23 ofthe system 200 of FIG. 2 are configured to use a shared memory 230.Shared memory 230 is memory space that contains items that can beaccessed by more than one virtual machine (and, typically, any virtualmachine configured to execute “like” application threads that is coupledto the shared memory 230).

Thus, whereas the prior art computing system 100 of FIG. 1 uses fewervirtual machines with larger local memory resources containing objectsthat are “private” to the virtual machine; the computing system 200 ofFIG. 2, by contrast, uses more virtual machines with less local memoryresources. The less local memory resources allocated per virtual machineis compensated for by allowing each virtual machine to access additionalmemory resources. However, owing to limits in the amount of availablememory space, this additional memory space 230 is made “shareable”amongst the virtual machines 123, 223, . . . M23.

According to an object oriented approach where each of virtual machines123, 223, . . . M23 does not have visibility into the local memories ofthe other virtual machines, specific rules are applied that mandatewhether or not information is permitted to be stored in shared memory230. Specifically, to first order, according to an embodiment, an objectresiding in shared memory 230 should not contain a reference to anobject located in a virtual machine's local memory because an objectwith a reference to an unreachable object is generally deemed “nonuseable”.

That is, if an object in shared memory 230 were to have a reference intothe local memory of a particular virtual machine, the object isessentially non useable to all other virtual machines; and, if sharedmemory 230 were to contain an object that was useable to only a singlevirtual machine, the purpose of the shared memory 230 would essentiallybe defeated.

In order to uphold the above rule, and in light of the fact that objectsfrequently contain references to other objects (e.g., to effect a largeprocess by stringing together the processes of individual objects;and/or, to effect relational data structures), “shareable closures” areemployed. A “closure” is a group of one or more objects where everyreference stemming from an object in the group that references anotherobject does not reference an object outside the group. That is, all theobject-to-object references of the group can be viewed as closing uponand/or staying within the confines of the group itself. Note that asingle object without any references stemming from can be viewed asmeeting the definition of a closure.

If a closure with a non shareable object were to be stored in sharedmemory 230, the closure itself would not be shareable with other virtualmachines, which, again, defeats the purpose of the shared memory 230.Thus, in an implementation, in order to keep only shareable objects inshared memory 230 and to prevent a reference from an object in sharedmemory 230 to an object in a local memory, only “shareable” (or“shared”) closures are stored in shared memory 230. A “shared closure”is a closure in which each of the closure's objects are “shareable.”

A shareable object is an object that can be used by other virtualmachines that store and retrieve objects from the shared memory 230. Asdiscussed above, in an embodiment, one aspect of a shareable object isthat it does not possess a reference to another object that is locatedin a virtual machine's local memory. Other conditions that an objectmust meet in order to be deemed shareable may also be effected. Forexample, according to a particular Java embodiment, a shareable objectmust also posses the following characteristics: 1) it is an instance ofa class that is serializable; 2) it is an instance of a class that doesnot execute any custom serializing or deserializing code; 3) it is aninstance of a class whose base classes are all serializable; 4) it is aninstance of a class whose member fields are all serializable; 5) it isan instance of a class that does not interfere with proper operation ofa garbage collection algorithm; 6) it has no transient fields; and, 7)its finalize ( )method is not overwritten.

Exceptions to the above criteria are possible if a copy operation usedto copy a closure into shared memory 230 (or from shared memory 230 intoa local memory) can be shown to be semantically equivalent toserialization and deserialization of the objects in the closure.Examples include instances of the Java 2 Platform, Standard Edition 1.3java.lang.String class and java.util.Hashtable class.

Cache Management Across Local and Shared Memory Resources

Note that the introduction of the shared memory 230 introduces theprospect of a shared cache 240. Thus, the architecture of FIG. 2includes both local memory level caches 126, 226, . . . M26 and a sharedmemory cache 240. FIG. 3 shows a depiction of a cache management service302 that can, for example, be added to the suite of services offered bya container 301 that an application thread runs in. A container is usedto confine/define the operating environment for the applicationthread(s) that are executed within the container. In the context ofJ2EE, containers also provide a family of services that applicationsexecuted within the container may use (e.g., (e.g., Java Naming andDirectory Interface (JNDI), Java Database Connectivity (JDSC), JavaMessaging Service (JMS) among others).

Different types of containers may exist. For example, a first type ofcontainer may contain instances of pages and servlets for executing aweb based “presentation” for one or more applications. A second type ofcontainer may contain granules of functionality (generically referred toas “components” and, in the context of Java, referred to as “beans”)that reference one another in sequence so that, when executed accordingto the sequence, a more comprehensive overall “business logic”application is realized (e.g., stringing revenue calculation, expensecalculation and tax calculation components together to implement aprofit calculation application).

FIG. 3 shows that more than one thread can be actively processed by thevirtual machine 323 depicted therein. It should be understood that, inaccordance with the discussion concerning FIG. 2, the number of threadsthat the virtual machine 323 can concurrently entertain should belimited (e.g., to some fixed number) to reduce the exposure to a virtualmachine crash. For example, according to one implementation, the defaultnumber of concurrently executed threads is 5. In a furtherimplementation, the number of concurrently executed threads is aconfigurable parameter so that, conceivably, for example, in a firstsystem deployment there are 10 concurrent threads per virtual machine,in a second system deployment there are 5 concurrent threads per virtualmachine, in a third system deployment there is 1 concurrent thread pervirtual machine. It is expected that a number of practical systemdeployments would choose less than 10 concurrent threads per virtualmachine.

The cache management service 302 is configured to have visibility intothe local memory cache 325 of the virtual machine 323, the shared memorycache 340 and one or more other storage resources 350 such as a databaseor file system used for storing persisted objects. Here, as will bedescribed in more detail below, different applications whose abstractcode (e.g., Java byte code in the case of Java) is executed by virtualmachine 323 can specially configure the cache management service 302 totreat its cached objects in accordance with specific guidelines.

According to various schemes, the cache manager 302 effectivelyconfigures regions of cache for the storage of objects in local cachememory 326 and/or in shared memory cache 340 according to differenttreatment policies. Multiple cache regions defining different cachetreatments may be established for a single application. Cached objectsplaced in local memory cache 326 may be conveniently utilized by thevirtual machine 323 associated with the local memory where local cache326 resides for quick processing by the application. By contrast, cachedobjects placed in shared memory cache 340 may be utilized by the localvirtual machine 323 as well as other virtual machines that havevisibility into the shared memory in which the shared memory cache 340is implemented.

FIG. 4 illustrates a more detailed perspective of an embodiment of thecache manager 302 of FIG. 3. Specifically, FIG. 4 illustrates theformation of multiple cache regions (cache region_1 410, cache region_2412, . . . cache region_N 414) that are controlled by cache manager 402.In one embodiment, a plurality of cache regions may be controlled bycache manager 402 for a single application. The cache regions may, forexample, be formed by commands executed by an application (e.g., app_1401) calling for the establishment of the cache regions themselves.

A cache region effectively determines the treatment that an object thatis stored in the cache region will receive. For example, cache region_1410 determines the treatment of object 460, while cache region_2 412determines the treatment of cached object 461. By comparison, object 460will receive different treatment than object 461 because of thedifferent treatment imposed by the different cache regions 410, 412.

For each cache region, in an embodiment, cache manager 402 implements astorage plug-in and an eviction policy plug-in. The storage plug-in maybe, in one embodiment, the actual piece of software or code thatexecutes the “get” and “put” operations for the objects stored accordingto the treatment determined by the associated cache region. That is, forexample, whether the object is placed in the local memory cache, theshared memory cache, or some other type of storage resource such as adatabase or file system for storing persisted objects. The evictionpolicy plug-in may be, in one embodiment, the actual piece of softwareor code that dictates the removal of an object from cache (e.g., whensome form of cache capacity threshold is exceeded).

In continuing from the example provided above, cache region_1 410defines the treatment of object 460 with storage plug-in_1 420 andeviction policy plug-in_1 421. Cache region_2 412 defines the treatmentof object 461 with storage plug-in_2 422 and eviction policy plug-in_2423. Cache region_N 414 is generally represented as having storageplug-in_N 424 and eviction policy plug-in_N 425. For simplicity ofdescription, each cache region is described as having only a singleobject that is treating according to the treatment determined by thecache region, but, it should be appreciated that any number of objectsmay be referenced by a particular cache region. Moreover, any objectstored in, copied from, written to, or removed from the shared memorycache 432 may be a single object; or, an object that is part of a sharedclosure where the shared closure itself is respectively stored in,copied from, written to, or removed from the shared memory cache 432.

As illustrated in FIG. 4, a storage policy plug-in of a cache region maydictate that an object stored in the local and/or shared cache memory becopied into a persisted storage space 440 (e.g., as part of the object'sremoval from the cache). One type of eviction process, referred to as“spooling,” initiates persistence of the object upon the object'seviction from cache. As such, the evicted object is written into deeperstorage space such as a hard disk file or a remote database 440. Anotheror related storage policy plug-in function may be used to perform a“write-through” process, in which a “put” of an object into cacheautomatically results in a copy of that object being directed to storagespace 440.

Until now, a cache region (e.g., cache region_1 410) has been generallydescribed as defining the treatment for a particular object, that is,for putting and/or getting an object to/from either the local memorycache and/or the shared memory cache. The following provides greaterdetail as to the different types of cache regions that may beimplemented by cache manager 402 for the treatment of objects as definedby its storage and eviction policy plug-ins. The different types ofcache management treatments are referred to as “flavors” or “cacheflavors”.

Cache Management Flavors

FIG. 5 illustrates an embodiment of the first cache region flavor 500,referred to as “Local”, which has object treatment behavior defined bycache region_1 511. Cache region_1 511 managed by cache manager 502includes a “local” flavor storage plug-in_1 520 and eviction policyplug-in_1 521 that together implement the treatment of objects that arecached within the first cache region. The local flavor is useable withnon-shareable objects that have no potential for storage into sharedmemory. The essence of the local flavor is that an object 560 is kept inlocal memory cache 530 and not shared memory cache 532; and, that hardreference(s) 570, 571 are made to the object 560 so that the object 560cannot be removed from the local memory cache 530 by a “garbagecollector.” A garbage collector, which is a well known process, removesobjects from local memory cache 530 (depending on the local memory usageand the type of references being made to the objects). Note that thegarbage collector is a background process that is different than theeviction policy processes instituted by the eviction policy plug-in 521.

As shown in FIG. 5, according to the “local” flavor, a first “hard”reference 570 is made by application_1 501 to object 560 (at least whilethe object 560 is actively being used by the application 501), and asecond “hard” reference 571 is made to the object 560 by storageplug-in_1 520. A particular type of reference to an object represents,in one embodiment, a relative difficulty level in removing an objectfrom the memory cache. A “hard” (or “strongly reachable”) referencedobject remains in the memory (i.e., the object is not removed from localmemory 530 by the garbage collector). A “soft” referenced object remainsin the memory until there is a danger of OutofMemoryError (e.g.,threshold level is exceeded in terms of available local memory space) orsome other algorithm (typically based on memory usage) used by thegarbage collector. A “weak” referenced object is removed by the garbagecollector regardless of the local memory's available space. A java VMimplementation is allowed however, to treat soft references like weakreferences (i.e., softly referred to objects are removed by the garbagecollector irrespective of memory usage).

Active removal of an object by the eviction policy plug-in (i.e.,eviction) ignores the referenced states of the object as describedabove. As such, hard referenced objects may be just as easily removed asweak referenced objects according to the policies set forth by theeviction policy plug-in 521. Here, note that storage plug-in 520 mayalso institute “spooling” and “write through” policies to deeperstorage. In an embodiment, a separate plug-in in cache region 511 (notshown) is used to interface with the deeper storage and is called uponas needed by storage plug-in 520 to institute spooling and write throughpolicies.

FIG. 6 illustrates an embodiment of a second cache region flavor 600referred to as “Local Soft.” The Local Soft flavor is similar to theLocal flavor of FIG. 5 but is different with respect to the referencesmade to the object 561 by the storage plug-in 522. In particular,storage plug-in_2 522 does not maintain a hard reference to object 561in local memory cache_1 530. Instead, a soft reference 573 isestablished. With a soft reference, according to one embodiment, object561 remains in local memory cache_1 530 until the eviction policyplug-in raises some type of memory availability concern, at which timean active eviction process is invoked by eviction policy plug-in_2 523.

When the active eviction process is invoked, soft reference 573 ischanged to a weak reference 574. Under this condition, object 561 may beremoved by a garbage collector if the application's hard reference nolonger exists (e.g., because the application is no longer actively usingthe object 561). That is, object 561 remains protected from removal bythe garbage collector as long as the application's hard reference 572 tothe object 561 is present, otherwise the object will be removed. Here,note that storage plug-in 522 may also institute “spooling” and “writethrough” policies to deeper storage. In an embodiment, a separateplug-in in cache region 512 (not shown) is used to interface with thedeeper storage and is called upon as needed by storage plug-in 522 toinstitute spooling and write through policies. In one embodiment, byinvoking the removal of object 560 from local memory cache_1 530 (eitherby active eviction or garbage collection), cache region_2 512 may alsoprovide for object 560 to be copied to deeper storage.

Before moving forward it is important to re-emphasize that objectsstored according to either of the local flavors discussed above may beof the non shareable type so as to be incapable of membership in ashared closure and storage into shared memory. Moreover, the applicationis apt to configure its different local cache regions such that objectsreceiving local flavor treatment are apt to be more heavily used (i.e.,some combination of the number of “get” and “put” accesses over time)than objects treated according to the Soft Local flavor.

FIG. 7 illustrates an embodiment of a third flavor 700 referred to as“Shared.” The “Shared” flavor is different from both the Local flavorsin that an object representation resides in shared memory cache 532 aspart of a shared closure. Under the “shared” flavor, when an application501 causes an object 562 a to be first “put” into local memory cache,the storage plug-in 524 also puts a copy 562 b of the object 562 a intothe shared memory cache 520. The application 501 places a hard reference575 to the local copy 561 a.

The application 501 is then free to use the local copy 562 a as a “workhorse” object. For each “put” operation made to the local copy 562 a,(e.g., to effectively modify the object's earlier content) the storageplug-in 524 updates/writes to the shared copy 562 b to reflect the “put”into the local memory cache 530. Note that because of the presence ofshared copy 562 b, a virtual machine other than the virtual machine thatis associated with the local memory within which local memory cache_1530 is implemented may copy the shared copy 562 b into its local memorycache (e.g., local memory cache 531) so as to create a third copy 562 cof the object. The third copy 562 c of the object can be used as a “workhorse” object for another application (not shown) that runs off of theother local memory cache 531. This other application will make a hardreference to this object 562 c as well (not shown). In one embodiment,storage plug-in 524 does not place any kind of reference to shared copy562 b because any shared closure is reachable in shared memory through akey name that uniquely identifies that shared closure; and moreover,shared closures are kept in shared memory until an applicationexplicitly calls a “delete” operation (i.e., no garbage collectionprocess is at work in shared memory at least for cached objects). Assuch, there is no need for any type of reference to a shared closureresiding in shared memory.

If the other application associated with local memory cache_2 531effectively modifies its local object 562 c (e.g., with a “put”operation), the storage plug-in for local memory cache_2 531 will createa “second version” 563 of shared object 562 b in shared memory cache 532that incorporates the modification made to local object 562 c. Accordingto an implementation, the storage plug-in 524 does not receive anyaffirmative indication of the existence of the new version but isinstead configured to look for new versions in shared memory (e.g., upona “put” or “get” operation) given the possibility of their existenceunder the shared flavor scheme. For instance, a “get” operation byapplication 501 will result in the reading of object 562 a and object563 by plug-in 524. Likewise, a “put” operation by application 501 canresult in the fetching of object 563 by plug-in 524 so that it ispossible to modify a local copy of the object 563 version. Here, notethat storage plug-in 524 may also institute “spooling” and “writethrough” policies to deeper storage. In an embodiment, a separateplug-in in cache region 513 (not shown) is used to interface with thedeeper storage and is called upon as needed by storage plug-in 524 toinstitute spooling and write through policies.

FIG. 8 shows another shared memory based flavor that may be referred toas “Shared Read-Only.” The essence of the Shared Read-Only approach isthat local copies do not exist (i.e., only an object 564 in sharedmemory cache 532 exists); and, no modification is supposed to be made tothe shared object under typical circumstances. The eviction policyplug-in 527 determines when the object 564 does not need to reside inshared memory cache 532 any longer.

In an extended embodiment, if a requirement to modify the object 564arises, the storage plug-in 526 associated with the application 501 thatdesires to make the modification creates an entirely new object andplaces it into the shared memory 532 as a second version 565.Subsequently, when object 564 is requested from shared memory 532 byanother application, the updated, second version 565 may also beretrieved. Here, note that storage plug-in 526 may also institute“spooling” and “write through” policies to deeper storage. In anembodiment, a separate plug-in in cache region 514 (not shown) is usedto interface with the deeper storage and is called upon as needed bystorage plug-in 526 to institute spooling and write through policies.

For either of the shared flavors discussed above, the storage plug-inmay be configured to control the size of the shared closures that arebeing entered into shared memory cache 532. Specifically, smaller sharedclosures may be “bundled” with other shared closures to form effectivelya data structure that contains multiple shared closures and that iseffectively treated as a single shared closure for copying operationsfrom shared memory cache 532 into local memory cache 530 (or viceversa). Here, a bundle may be created simply by ensuring that eachshared closure in the bundle is associated through a reference toanother shared closure in the bundle.

By increasing bundle size, overhead associated with copying objects backand forth between shared memory and local memory is reduced in certaincircumstances, particularly, environments where many smaller sharedclosures are to be sent between shared memory and local memory at aboutthe same time. Here, by bundling them, all shared closures caneffectively be transported between shared memory and local memory by asingle transfer process.

Storage Plug-In Programming Models

Until now, the storage plug-in for a particular cache region has beengenerally described as defining the cache storage treatment of one ormore objects associated with the cache region. The storage plug-in maybe, in one embodiment, the actual piece of software or code thatexecutes various operations (e.g., “get” or “put”) for objects storedaccording to the treatment determined by the associated cache region.FIG. 9 illustrates a more detailed perspective of a possibleimplementation for a single cache region 602. Recall that multiple cacheregions may be established for a single application. Cache region 602 isshown having storage plug-in 603 and eviction policy plug-in 610.

Storage plug-in 603, in one embodiment, is logically represented asbeing capable of performing several functions, including Key ObjectManipulation 604, Key Attribute Manipulation 605, Group Manipulation606, Key Set Operations 607, Key System Attribute Manipulation 608, andObject Size Information 609. Several functionalities are also associatedwith eviction policy plug-in 610. These functionalities include Sorting611, Eviction Timing 612, and Object Key Attribution 613. The variousfunctionalities of eviction policy plug-in 610, which also define atreatment of objects in local memory cache 630 and shared memory cache632, are described in greater detail further below with respect to FIGS.13 a,b-15. One, all, or a combination of these functionalities may beassociated with each object that is handled according to the treatmentdefined by cache region 602. Again, exemplary discussing is provided inthe context of a single object. But, it should be understood that atleast with respect to the treatment of objects cached in shared memory,such objects may also be in the form of a shared closure.

Key Object Manipulation 604 is a storage plug-in function that relatesto the “get” and “put” operations for an object. For example, a “get”operation retrieves a particular object from local cache memory 630and/or shared memory cache 632 depending on the “flavor” of the plug-in(consistent with the different caching flavors described in thepreceding section). A “put” operation places a copy of an object intolocal memory cache 630 and/or shared memory cache 632 (again, consistentwith the specific “flavor” of the plug-in). For each object associatedwith a cache region, an object name may be assigned to each object. Inturn, each object name may correspond to a unique key value. Oneembodiment of this organizational structure is illustrated in FIG. 10.

Referring to FIG. 10, cache region 602 includes a cache group_1 620associated with N objects 670, 671, . . . 672. It is important to pointout that multiple groups of objects (or “object groups”) may beestablished per cache region (i.e., FIG. 10 only shows one group butmultiple such groups may exist in cache region 602). As will bedescribed in more detail below, assignment of objects into a groupallows for “massive” operations in which, through a single command fromthe application, an operation is performed with every object in thegroup.

Each object of cache group_1 620 is associated with a unique key. Thatis, for example, Key_1 640 is associated with object 670, key_2 641 isassociated with object 671, and key_N is associated with object 672.Each key is a value (e.g., alphanumeric) that, for instance, in oneembodiment, is the name of the object. In an embodiment, the key for anobject undergoes a hashing function in order to identify the numericaladdress in cache memory where the object is located.

As such, the Key Object Manipulation functionality 604 of storageplug-in 603 utilizes the key associated with an object to carry out“put” and “get” operations on that object. For simplicity, only a localmemory cache 635 is considered (e.g., the storage plug-in may be a“local” or “soft local” flavor).

As an example, object 670 may have the key “Adam” in simple text form.An application (e.g., application_1 601 of FIG. 9) provides the inputfor a “put” operation of object 670 which may take the form of [PUT,ADAM] in cache. The key, “Adam,” undergoes a hashing function by storageplug-in 603 to generate the cache address where object 670 is to bestored. The key object manipulation “put” functionality of storageplug-in 603 completes the “put” operation by'writing object 670 to localmemory cache 630 at the address described provided by the hashingfunction.

A feature of the Key Object Manipulation 604 functionality is that anapplication does not need to know the exact location of a desiredobject. The application merely needs to reference an object by its keyonly and the Key Object Manipulation 604 functionality of the storageplug-in is able to actually put the object with that key into the localmemory cache 630.

A “get” operation may be performed by the Key Object Manipulation 604functionality in a similar manner. For example, object 671 may have thename “Bob.” An application (e.g., application_1 601 of FIG. 9) providesthe input for the “get” operation of object 671 which may take the formof, [GET BOB] from cache. The key, “Bob,” undergoes a hashing functionby storage plug-in 603 to determine the numerical address where object671 is stored in local memory cache 630. The Key Object Manipulation 604functionality of storage plug-in 603 completes the “get” operation bycopying or removing object 671 to some other location in local memory635 outside the cache.

Key Attribute Manipulation 605 is a functionality that relates todefining or changing particular attributes associated with an object.Here, each object has its own associated set of “attributes” that, inone embodiment, are stored in cache address locations other than that ofthe object itself. Attributes are characteristics of the object'scharacter and are often used for imposing appropriate treatment upon theobject. Examples of attributes include shareable/non-shareable andtime-to-live (an amount of time an object is allowed to stay in a cacheregion before being evicted). As each cached object is associated with akey, an object's attributes may also be associated with the key.

Thus, as depicted in FIG. 10, Key_1 640, which is the key for object670, is also associated with the collection of attributes_1 680 forobject 670. Key_2 641, which is the key for object 671, is alsoassociated with the collection of attributes_2 681 for object 671. Notethat the attributes 680-682 are also stored in local memory cache 630(but are not drawn in FIG. 10 for illustrative simplicity). As will bedescribed in more detail below, in an embodiment, the key AttributeManipulation function 605 performs a first hashing function on the keyto locate the collection of attributes for the object in cache; and,performs a second hashing function on a specific type of attributeprovided by the application to identify the specific object attributethat is to be manipulated (e.g., written to or read).

The Key System Attribute Manipulation 608 allows for system attributes(i.e., system level parameters) to be keyed and manipulated, and,operates similarly to the key attribute manipulation 605.

Group Manipulation 606 is a storage plug-in functionality that allowsfor “put” or “get” manipulation of all the objects within a particulargroup. By specifying the group name for a group of objects, theapplication may retrieve (“get”) all the objects within that group. Inan embodiment, the keys for a particular group are registered with thestorage plug-in 603. As such, a group name that is supplied by theapplication is “effectively” converted into all the keys of the objectsin the group by the storage plug-in 603. For example, application_1601may run a “get” operation for cache group_1 620. By using the name ofcache group_1 620 as the input, each of keys key_1 640, key_2 641, . . .key_N 642 are used by the storage plug in cache of keys to perform a“get” operation.

FIG. 11 illustrates a block diagram 900 of one embodiment of a “get”operation using Group Manipulation 606 functionality and is described inassociation with FIG. 9 and FIG. 10. This functionality is particularlyuseful in scenarios involving “massive” operations in which all objectsfrom a particular group are to be affected. Application_1 601 firstspecifies 701 the group name (e.g., the name of cache group_1 620)needed for the operation and the “get” operation itself. In response,the Group Manipulation 606 functionality of the storage plug-in 603retrieves 702 all the objects in the group by using the key for theobject the group. The “get” operation ends 703 when there are no morekeys to use.

The Object Size Information function 609 causes the storage plug-in 603to calculate the size of an object (e.g., in bytes). Here, theapplication supplies the key of the object whose size is to becalculated and specifies the object size information function 609.Combined with a Group Manipulation function, the Object Size Informationfunction 609 enables storage plug-in 603 to calculate the size of anentire group. The Key Set Operations function 607 is used to performspecific operations with the keys themselves (e.g., return to theapplication all key values in a group specified by the application).

As discussed above, each object may have a collection of attributes(e.g., shareable/non-shareable, time-to-live, etc.). In one embodiment,these attributes may be organized in local memory cache to be accessedby an application in a manner similar to the retrieving of an objectwith a “get” operation described above with respect to the Key ObjectManipulation 604 function. In one embodiment, a series of hashingoperations may be performed to retrieve one or attributes of aparticular object.

FIG. 12 illustrates a more detailed perspective of an approach foraccessing an attribute associated with a particular object. As anextension of the example provided above for object 670 (the getoperation for the “Adam” object), FIG. 12 illustrates an attributestable 655 for object 670 organized in local memory cache 630. In oneembodiment, attributes table 655 may be within a region of local memorycache in which a specific address value (e.g., address_1 660, address_2662, . . . address N 664) is associated with a specific attribute value(e.g., value_1 661, value_2, 663, . . . value_N 665).

A “get” operation for a particular attribute of an object may be carriedout in the following manner. Application_1 601 specifies: 1) theoperation 658 (e.g., “get”); 2) the key for the object 668 (e.g.,“ADAM”); and, 3) the applicable attribute 678 (e.g.,“SHAREABLE/NON-SHAREABLE”). As discussed above with respect to FIG. 10,a collection of attributes (e.g., attributes table 655) may beassociated with a particular object. In the approach of FIG. 12, thetable 655 for a particular object 670 is made accessible with theobject's key. For example, as illustrated in FIG. 12, the key 688(“Adam”) for object 670 undergoes a first hashing function (i.e., hash_1650), that, in consideration of the operation pertaining to anattribute, causes the numerical address in local memory cache 630 whereattributes table 655 is located to be identified.

A second hashing function (i.e., hash_2 651) is performed using thedesired attribute 678 (e.g., SHAREABLE/NON-SHAREABLE) as the key. Thehash_2 651 hashing function identifies the particular numerical addressof the particular attribute of attributes table 655 that is to beaccessed. For example, if the Shareable/Non-shareable attribute valuecorresponds to value_2 663, the alphanumeric “name” of the attribute(e.g., “Shareable/Non-shareable”) would map to address_2 662 ofattributes table 655.

Eviction Policy Programming Models

Caches, either local or shared, have limited storage capacities. Assuch, a cache may require a procedure to remove lesser used objects inorder, for example, to add new objects to the cache. Similar to astorage plug-in being designed to impose certain storage treatment(s) onan object, the eviction policy plug-in provides various functionalitiesfor the active removal of an object from cache. As briefly discussedabove with respect to FIG. 9, and as again provided in FIG. 13A,eviction policy plug-in 610 is logically represented as being cable ofperforming several functions, including object sorting 611, evictiontiming 612, and object key attribution 613.

Referring to FIG. 13A, sorting component 611 is type of a queuingfunction that effectively sorts objects stored in a cache region so thata cached object that is most appropriate for eviction can be identified.Different sorting component types that each enforces a different sortingtechnique may be chosen from to instantiate a particular eviction policywith plug-in 606. That is, in one embodiment, there are different“flavors” of object sorting that may be selected from, and, one of thesemay be used to impose treatment on, for instance, an entire cacheregion. In other embodiments, multiple object sorting components(conceivably of different flavors) may be established per cache regione.g., one mutation notification per cache group).

To the extent the sorting component 611 can be viewed as a componentthat chooses “what” object should be removed from cache, the evictiontiming component 612 is a component that determines “when” an objectshould be removed from cache. Different flavors for eviction timingcomponents may also exist and be chosen from for implementation.Typically, a single eviction timing component is instantiated per cacheregion; but, conceivably, multiple eviction policy components may beinstantiated as well (e.g., one per cache group). The object keyattribution 613 component enables the involvement of certain objectattributes (e.g., object size) in eviction processes.

For simplicity, the remainder of this detailed description will bewritten as if an eviction policy plug-in applies to an entire cacheregion.

FIG. 13B illustrates a more detailed perspective of various types ofsorting components 611 that may be chosen for use within a particulareviction policy plug-in 603. In one embodiment, four types of queues maybe implemented by sorting component 611: 1) a Least Recently Used (LRU)queue 617; 2) a Least Frequently Used (LFU) queue 618; 3) a size-basedqueue 619; and, 4) a First In First Out (FIFO) queue 621. In oneembodiment, the different types of sorting techniques queue the keysassociated with the cache region's cached objects. The identification ofa key that is eligible for eviction results in the key's correspondingobject being identified for eviction. In the case of shared closures,various approaches are possible. According to a first approach, sortingis performed at the object level, that is, keys are effectively sortedwhere each key represents an object; and, if a particular objectidentified for eviction happens to be a member of a shared closure thatis cached in shared memory cache, the entire shared closure is evictedfrom shared memory cache (e.g., with a “delete” operation). According toa second approach, if an object is a member of a shared closure, a keyfor the shared closure as a whole is sorted amongst other keys. Ineither case, identifying a “key” that is eligible for eviction resultsin the identifying of an object for eviction (where, in the case ofshared closure, an object and all its shared closure member objects areidentified for eviction).

According to the design of the LRU queue 617, objects cached in a cacheregion that are accessed least recently (e.g., through either a “get” or“put” operation) are discarded first. LRU queue 617 is represented witha vertical ordering structure for multiple keys (e.g., key_1 655, key_2656, . . . key_N 657). Essentially, the top of the queue represents keysfor objects that have been used most recently, and, the bottom of thequeue represents keys for objects that have been used least recently.According to one implementation of LRU queue 617, the objectcorresponding to the key at the very bottom would be next evicted.Removal of a key from the bottom of the queue triggers the eviction ofthat key's corresponding object from cache.

Here, any time an object is accessed (e.g., by way of a “get” or “put”operation), the key corresponding to that object is positioned at thevery top of LRU queue 617. As illustrated by the position of key_1 655,the object associated with key_1 655 is the most recently accessedobject. If, however, in a following operation an application (e.g.,application_1 601) accesses the object associated with key_2 656, then,key_2 656 would be repositioned above key_1 655 in the LRU queue 617.

At any given instant of time, the key whose object has spent the longestamount of time in the cache region without being accessed will reside atthe bottom of the queue. As such, when the moment arises to remove anobject from the cache region, the object whose key resides at the bottomof the queue will be selected for removal from the cache region.

LFU queue 618 is an eviction policy in which cached objects accessedleast frequently (e.g., through either a “get” or “put” operation),based on a counter, are discarded first. Each key for an object may havean associated counter that measures or keeps track of the number oftimes the object is accessed (e.g., the counter for the object's key isincremented each time the object is accessed). In one embodiment, thecounter value may be an “attribute” for the object as describedpreviously.

As with LRU queue 617, LFU queue 618 is represented with a verticalordering structure for multiple keys (e.g., key_1 665, key_2 666, . . .key_N 667). The top of the queue represents keys for objects that havethe highest counter value, and the bottom of the queue represents keysfor objects with the lowest counter value. Here, over the course oftime, those keys whose corresponding objects are accessed morefrequently than other cached objects will be “buoyant” and reside nearthe top of the queue; while, those keys whose corresponding objects areaccessed less frequently than the other objects in the cache region will“sink” toward the bottom of the queue.

At any instant of time, the key whose corresponding object has been usedless than any other object in the cache region will be at the bottom ofthe queue. Thus, according to one implementation of LFU queue 618, theobject corresponding to the key at the very bottom would be nextevicted, because that object has the lowest counter value (i.e., lowestfrequency of use). Removal of the key from the bottom of the queuetriggers the eviction of that key's corresponding object from the cacheregion. Note that the counters for all the keys may be resetperiodically or with each entry of a newly cached object in order toensure that all the counter values can be used as a comparativemeasurement of use.

Size-based queue 619 is an eviction policy in which cached objects areprioritized according to size (e.g., the number of total bytes for theobject). As such, object size may be another object attribute. The keysfor objects in size-based queue 619 are shown arranged vertically withthe smallest objects positioned near the top of the queue and keys forthe largest objects positioned near the bottom of the queue. Accordingto one implementation of size-based queue 619, the object correspondingto the key at the very bottom would be evicted first, because thatobject consumes the most amount of cache region space, and itssubsequent removal would result in the most amount of free cache regionspace recovered (amongst all the objects that are cached in the cacheregion).

FIFO queue 621 is an eviction policy in which cached objects are removedaccording to the order that they are placed in the cache relative to oneanother. In one embodiment, when an eviction moment arises, the firstcached object eligible for eviction corresponds to the object that hasspend the most time in the cache, followed by the next oldest object,and so on. FIFO queue 621, illustrated in FIG. 13 b, is also depictedwith a vertical ordering structure for key_1 655, key_2 656, . . . key_N657, with key_1 655 corresponding to the oldest object (i.e., the firstobject placed in the cache) and key_N 677 corresponding to the newestobject (i.e., the most recent object placed in the cache). When aneviction process is triggered, the object for key_1 675 would be thefirst for removal. Unlike the other types of queues described above(assuming the size of an object can change in respect of size-basedqueue 619), there is no possibility for any rearrangement of the keyorder in FIFO queue 621. The keys are maintained in the order they areadded to the cache, regardless of frequency, counter value, or size.

Referring back to FIG. 13A, the eviction timing component 612 is afunctionality that determines when an object should be removed from acache region. FIG. 14 illustrates a detailed graph of one type ofeviction timing approach. The vertical axis represents the total numberof objects in the cache as represented by the total number of keys inthe queue associated with the applicable object sorting approach. Thehorizontal axis represents time (e.g., in milliseconds). Countallocation 648 represents the “targeted” maximum number of keys in thequeue, which, in turn, corresponds to the targeted maximum number ofallowable objects in the cache region.

In one embodiment, three threshold levels may be established for thecache region. A first threshold level, threshold_1 645, corresponds to alevel at which the eviction of a key from the sorting queue occurs on atimely basis. For example, when the count exceeds threshold_1 645 (butnot threshold_2 646), a key is evicted from the sorting queue everymillisecond until the total count falls below threshold_1 645. In oneembodiment, no active eviction occurs for count levels below threshold_1645.

A second threshold level, threshold_2 646, corresponds to a level abovewhich eviction of a key occurs on each entry into the cache of a newlycached object. That is, with each new addition of an object into cache,the key at the bottom of the applicable sorting queue is removed fromthe queue resulting in its corresponding object's eviction from cache.With this approach, the population of the cache region should remainconstant in the absence of objects being removed from the cache regionby processes other than eviction (such as deletion and/or garbagecollection and/or attribute based as described below with respect toObject Key Attribution). With processes other than eviction, the cacheregion population may fall below threshold_2 646 after the threshold hasbeen crossed.

A third threshold level, threshold_3 647, corresponds to a level equalto the targeted maximum allocation 648 for the cache region. When thislevel is exceeded, keys are evicted from the sorting queue until, in oneembodiment, the total count of keys decreases to threshold_3 647 (orjust beneath threshold_3 647). Note that this approach contemplates thepopulation of the cache region exceeding its “targeted” maximumallocation for some reason.

Either of the eviction timing techniques may be used with the LRU 617LFU 618 or FIFO 619 sorting technique. FIG. 15, by contrast, illustratesa detailed graph of another type of eviction timing technique that is tobe used with the size based 619 sorting technique. In this embodiment,the vertical axis represents the total amount of consumed memory spaceof the cache. The horizontal axis represents time (e.g., inmilliseconds). Size allocation 689 represents the maximum “targeted”allocated memory capacity of the cache region in terms of size (e.g.,bytes).

In one embodiment, threshold_1 685, threshold_2 686, and threshold_3 687have similar properties with threshold_1 645, threshold_2 646, andthreshold_3 647, respectively. The only difference is that the memoryconsumption of the cache region (through the caching of its cachedobjects) triggers the crossing of the various thresholds.

Referring again back to FIG. 13A, Object Key Attribution 613 is afunctionality that allows for the eviction of objects based on specificattributes that may be user-defined, system-defined, or otherwisecustomizable. For example, objects may be evicted on a Time-To-Live(TTL) basis in which case an object's key is pulled from the sortingqueue (regardless of where it is located within the queue) if the objectresides in the cache region for more than an amount of time set by theTTL attribute. Another attribute based eviction policy is AbsoluteEviction Time (AET). In the case of AET, an actual time is set (e.g.,12:00 AM). If the object resides in the cache region after this time theobject is evicted from the cache region.

Also, in the case of size based eviction policies, each objects size maybe found in its attribute table.

Cache Management Library

The preceding discussions revealed that, referring to FIG. 16, a cachemanagement library 1601 containing various plug-ins may be used to helpbuild cache regions that impose various forms of object/shared closuretreatment. Specifically, the Local, Local Soft, Shared and Shared ReadOnly storage plug-ins 1602_1, 1602_2, 1602_3, 1602_4 may be part of acollective storage plug in library 1602; and, the LRU, LFU, Size Basedand FIFO sorting plug-in components 1603_1, 16032, 1603_3, 1603_4 may bepart of a collective sorting plug-in component library 1601.

Here, definition of a specific cache region is effected by selecting1604 a storage plug-in from the storage plug-in part 1602 of the cachemanagement library 1601 and by selecting 1605 a sorting componentplug-in from the sorting component plug-in part 1603 of the cachemanagement library 1601. For each new cache region to be implemented,another iteration of the selection processes 1604, 1605 is performed.Thus, if a single application were to establish multiple cache regions,the configuration for the application would entail running throughselection processes 1604, 1605 for each cache region to be implemented.

Distributed Cache Architecture

As discussed above with respect to FIG. 4, a storage policy plug-in of acache region may dictate that an object stored in the local and/orshared cache memory be copied into deeper storage space 440 (e.g., apersisted database, in response to an object's removal from the cache).In one embodiment, the storage of a particular object into deeperstorage allows for the “sharing” of that object on a much larger scale(e.g., between different computing systems or application servers). Forexample, an object commonly used by a cluster of application servers maybe written to a persisted database for retrieval by any physicalmachine.

In another example, a first computing system having a first virtualmachine may crash during the course of running operations with a numberof objects. If the objects are stored in a persisted database, a secondvirtual machine from a second computing system may be able to restorethe operations that were running on the first computing system, usingthe same objects retrieved from the persisted database.

FIG. 17 and block diagram 1000 of FIG. 18, taken together, illustrateone method of preserving an object's cached status between two computingsystems. Application_1 803, running on computing system_1 801, specifiesa “PUT” operation 830 for object 850 a into local memory cache_1 811 orshared memory cache_1 805. In one embodiment, the “PUT” operation mayinvolve the various functionalities of a storage plug-in described abovefor a cache region by cache manager_1 804. Object 850 a is genericallyrepresented but in one embodiment object 850 a may be a group ofobjects, and in another embodiment may be objects contained within ashared closure. Object 850 a is then persisted 831 in database 820 thatis visible to other computing systems, including computing system_2 802.

In one embodiment, a Structured Query language (SQL), or SQL-likecommand statement may be used to write a serialized version of object850 b into database 820. (In FIG. 17, the de-serialized object isreferenced as 850 a, and the serialized object is referenced as 850 b).In alternate embodiments, other known database languages may be used towrite object 850 b into database 820. Upon successful writing of object850 b in database 820, a notification “statement of success” is sent 832to cache manager_1 804. Along with the success notification statement,the key for object 850 b may also be sent to cache manager_1 804, where,according to a further implementation, the key is in a de-serializedform. Object keys have been discussed in detail above with respect toFIGS. 10-12.

Upon receiving the success notification and the de-serialized key forobject 850 b, cache manager_1 804 serializes 833 the key for object 850b and sends the serialized key 834 across a network 806 to computingsystem_2 802. Cache manager_2 808 receives the serialized key for object850 b and then de-serializes the key, 835. The de-serialized key maythen be registered with a storage plug-in associated with cachemanager_2 808.

When application_2 807 running on computing system_2 802 requests 837object 850 b, the de-serialized object key that is registered with cachemanager_2 808 is used to retrieve 838 the serialized object 850 b fromdatabase 820 at computing system_2 802. The serialized object 850 b maythen be de-serialized 839 by cache manager_2 808. The de-serializedobject 850 a may then be saved in local memory cache_2 809 and/or sharedmemory cache_2 810.

Closing Comments

Processes taught by the discussion above may be performed with programcode such as machine-executable instructions which cause a machine (suchas a “virtual machine”, a general-purpose processor disposed on asemiconductor chip or special-purpose processor disposed on asemiconductor chip) to perform certain functions. Alternatively, thesefunctions may be performed by specific hardware components that containhardwired logic for performing the functions, or by any combination ofprogrammed computer components and custom hardware components.

An article of manufacture may be used to store program code. An articleof manufacture that stores program code may be embodied as, but is notlimited to, one or more memories (e.g., one or more flash memories,random access memories (static, dynamic or other)), optical disks,CD-ROMs, DVD ROMs, EPROMs, EEPROMs, magnetic or optical cards or othertype of machine-readable media suitable for storing electronicinstructions. Program code may also be downloaded from a remote computer(e.g., a server) to a requesting computer (e.g., a client) by way ofdata signals embodied in a propagation medium (e.g., via a communicationlink (e.g., a network connection)).

FIG. 19 is a block diagram of a computing system 1900 that can executeprogram code stored by an article of manufacture. It is important torecognize that the computing system block diagram of FIG. 19 is just oneof various computing system architectures. The applicable article ofmanufacture may include one or more fixed components (such as a harddisk drive 1902 or memory 1905) and/or various movable components suchas a CD ROM 1903, a compact disc, a magnetic tape, etc operable withremovable media drive 1904. In order to execute the program code,typically instructions of the program code are loaded into the RandomAccess Memory (RAM) 1905; and, the processing core 1905 then executesthe instructions. The processing core 1906 may include one or moreprocessors and a memory controller function. A virtual machine or“interpreter” (e.g., a Java Virtual Machine) may run on top of theprocessing core 1806 (architecturally speaking) in order to convertabstract code (e.g., Java byte code) into instructions that areunderstandable to the specific processor(s) of the processing core 1906.

It is believed that processes taught by the discussion above can bepracticed within various software environments such as, for example,object-oriented and non-object-oriented programming environments, Javabased environments (such as a Java 2 Enterprise Edition (J2EE)environment or environments defined by other releases of the Javastandard), or other environments (e.g., a .NET environment, a Windows/NTenvironment each provided by Microsoft Corporation).

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

1. A system comprising: a cache manager to: create a first cache regionassociated with an application, the application running on a pluralityof virtual machines, a first one of the plurality of virtual machineshaving a local memory cache that is private to the first one of theplurality of virtual machines and a shared memory cache that is sharedby the first one of the plurality of virtual machines and a second oneof the plurality of virtual machines; and associate the first cacheregion with a first storage policy, the first storage policy specifyingan object to be stored in the first cache region is to be stored in thelocal memory cache and that a garbage collector is not to remove theobject from the local memory cache.
 2. The system of claim 1, whereinthe first storage policy further specifies that an eviction of theobject from the first cache region is to result in a storage of theobject in a storage resource having a higher latency than the localmemory cache.
 3. The system of claim 2, wherein the storage resource isa local memory associated with the local memory cache.
 4. The system ofclaim 1, wherein the cache manager is further to: receive a request fromthe application to store the object in the first cache region; inresponse to the receiving of the request from the application to storethe object, store the object in the local memory cache and establish ahard reference to the object, an identifying of the hard reference bythe garbage collector to prevent the garbage collector from removing theobject from the local memory cache regardless of a lack of a use of theobject by the application.
 5. The system of claim 1, wherein the cachemanager is further to: create a second cache region associated with theapplication; and associate a second storage policy with the secondstorage region, the second storage policy specifying that an additionalobject to be stored in the second cache region is to be stored in thelocal memory cache and that a lack of a use of the additional object bythe application is to enable the garbage collector to remove theadditional object from the local memory cache.
 6. The system of claim 5,wherein the cache manager is further to: receive a request from theapplication to store the additional object in the second cache region;and in response to the receiving of the request from the application tostore the additional object, store the additional object in the localmemory cache and establish a soft reference to the additional object, anidentifying of the soft reference by the garbage collector enabling thegarbage collector to remove the additional object from the local memorycache unless the application has a hard reference to the additionalobject, an identifying of the hard reference by the garbage collectorpreventing the garbage collector from removing the additional objectfrom the local memory cache.
 7. A method comprising: creating a firstcache region associated with an application, the application running ona plurality of virtual machines, a first one of the plurality of virtualmachines having a local memory cache that is private to the first one ofthe plurality of virtual machines and a shared memory cache that isshared by the first one of the plurality of virtual machines and asecond one of the plurality of virtual machines; and associating thefirst cache region with a first storage policy, the first storage policyspecifying that a first copy of an object to be stored in thefirst-cache region is to be stored in the local memory cache and that asecond copy of the object to be stored in the first cache region is tobe stored in the shared memory cache.
 8. The method of claim 7, furthercomprising: receiving a request from the application to retrieve theobject, the object having been stored in the first cache region; and inresponse to the receiving of the request from the application toretrieve the object, detecting that a second version of the object isnewer than a first version of the object, the second version of theobject being stored in the shared memory cache and the first version ofthe object being stored in the local memory cache; and in response tothe detecting, providing the second version of the object to theapplication.
 9. The method of claim 8, wherein the providing of thesecond version of the object comprises copying the second version of theobject from the shared memory cache to the local memory cache.
 10. Themethod of claim 9, wherein the providing of the second version of theobject further comprises enabling the application to establish a hardreference to the second version of the object in the local memory cache,an identifying of the hard reference by a garbage collector preventingthe garbage collector from removing the second version of the objectfrom the local memory cache.
 11. The method of claim 7, wherein thefirst storage policy further specifies that an eviction of the objectfrom the local memory cache is to result in a storage of the object in astorage resource having a higher latency than the local memory cache.12. The method of claim 11, wherein the storage resource is a sharedmemory associated with the shared memory cache.
 13. The method of claim7, further comprising: creating a second cache region associated withthe application; and associating the second cache region with a secondstorage policy, the second storage policy specifying that a firstversion of an additional object to be stored in the second cache regionis to be stored in the shared memory cache and that the first version ofthe additional object is not to be modified.
 14. The method of claim 13,further comprising: receiving a request from the application toincorporate a modification into the additional object, the additionalobject having been stored in the second cache region; and in response tothe receiving of the request from the application to incorporate themodification, incorporating the modification into a second version ofthe additional object and storing the second version of the additionalobject in the shared memory cache.
 15. A machine-readable storage mediumcomprising instructions that, when executed by a processor of a machine,causes the machine to perform a method comprising: creating a cacheregion associated with an application, the application running on aplurality of virtual machines, a first one of the plurality of virtualmachines having a local memory cache that is private to the first one ofthe plurality of virtual machines and a shared memory cache that isshared by the first one of the plurality of virtual machines and asecond one of the plurality of virtual machines; and associating thecache region with a storage policy, the storage policy specifying that afirst shared closure stored in the cache region is to be bundled with asecond shared closure stored in the cache region based on a size of thefirst shared closure and a size of the second shared closure nottransgressing a size threshold, the first shared closure being a firstplurality of objects, each of the first plurality of objects shareableby the plurality of virtual machines and not referencing an objectoutside the first plurality of objects, the second shared closure beinga second plurality of objects, each of the second plurality of objectsshareable by the plurality of virtual machines and not referencing anobject outside the second plurality of objects.
 16. The machine-readablestorage medium of claim 15, the method further comprising: receiving arequest from the application to retrieve the first shared closure fromthe cache region; in response to the receiving of the request from theapplication to retrieve the first shared closure, detecting that thefirst shared closure is bundled with the second shared closure; and inresponse to the detecting, providing the application with the firstshared closure and the second shared closure.
 17. The machine-readablestorage medium of claim 16, wherein the providing of the applicationwith the first shared closure and the second shared closure comprisestransporting the first shared closure and the second shared closure fromthe shared memory cache to the local memory cache using a singletransfer process.
 18. The machine-readable storage medium of claim 15,wherein the bundling of the first shared closure with the second sharedclosure is also based on a determination that a number of sharedclosures transported between the shared memory cache and the localmemory cache within a time period transgresses a quantity threshold. 19.The machine-readable storage medium of claim 15, the method furthercomprising associating the cache region with an eviction policy, theeviction policy specifying that an object stored in the cache region isto be evicted based on an attribute of the object regardless of aposition of the object in a sorting queue associated with the evictionpolicy.
 20. The machine-readable storage medium of claim 15, wherein theattribute is one of a time-to-live attribute and an absolute evictiontime attribute.